Some vehicles are equipped with engines that use a start-stop operation to reduce fuel consumption. The start-stop operation allows the engine to be automatically shut-off when the vehicle comes to a stop and the transmission is in drive. For example, an engine in a vehicle using the start-stop operation is shut-off when the vehicle is temporarily stopped at a stop light. The vehicle re-starts the engine once the driver presses the gas pedal to commence traveling when the stop light turns green. By shutting off the engine when the vehicle is stopped, fuel consumption can be reduced.
Starting the engine with the transmission in drive, however, creates some problems. When in drive, the transmission has to be ready with sufficient pressure in its elements to support engine torque as the engine starts back up. If the transmission is not ready, then the engine is not be able to transfer torque to the transmission and the vehicle hesitates after the driver presses the gas pedal.
This hesitation is a result of the automatic transmission's design. Automatic transmissions use hydraulically driven pistons with return springs to operate the clutches that are used to engage the transmission's gears. When the engine of a vehicle is turned off, all of the hydraulic pressure in the transmission is lost and the pistons' return springs stroke the pistons away from the clutches. As a result, before the pistons can apply pressure to the clutches so that the transmission can support the engine torque, the hydraulic system must provide sufficient pressure to the pistons to overcome the force of the return springs and stroke the pistons into contact with the clutches.
To reduce this hesitation, vehicles that employ the start-stop operation use an electric pump to maintain some hydraulic pressure in the pistons. This allows the transmission to create the pressure necessary to engage the clutches associated with first gear more quickly and reduce hesitation of the vehicle after a start-stop operation. These electric pump systems, however, are costly and require additional logic, valves, and sensors to implement. What is needed is a design that allows a transmission to quickly create the pressure necessary to engage the clutch associated with first gear during a start-stop operation without changing the operation of the transmission during normal use and without the need for the costly, logic, valves and sensors currently used.